The present invention generally relates to a method for manufacturing a semiconductor device, and more specifically, to a method for manufacturing a semiconductor device that improves the refresh characteristic of a memory device.
A threshold voltage (Vt) is the most important parameter in the manufacture of a transistor. The threshold voltage depends on the thickness of the gate oxide film, the channel doping concentration, the oxide charge, and the gate material. The threshold voltage shows phenomena that are not identical with theoretical values as a high integration of the semiconductor device.
Due to the high integration of the semiconductor device, a device that can be operated at a lower operating voltage ranging from 1V to 2V with an improved speed is required. The threshold voltage also requires a lower voltage. However, it is difficult to control a device due to the short channel effect when the threshold voltage is decreased. The short channel effect causes a Drain Induced Built-in Leakage (DIBL) by hot carriers. Although various attempts have been made to reduce the short channel effect, a satisfactory solution has not been found to prevent increasing the short channel effect due to the high integration of a semiconductor device.
For example, the short channel effect has not been addressed by regulation of the doping concentration. The current solutions include a method for forming an ion-implant channel and a Super Steep Retrograde Channel (SSR) through vertically abrupt channel doping, a laterally abrupt channel doping method, and a method for forming a channel having a halo structure through a large angle tilt implant.
FIGS. 1a to 1d are plane views illustrating a conventional method for manufacturing a semiconductor device. Referring to FIG. 1a, a pad nitride film 110 is formed over the semiconductor substrate 100. The pad nitride film 110 and the semiconductor substrate 100 are etched to form a trench 120.
Referring to FIGS. 1b and 1c, an oxide film 130 is formed at sidewalls of the trench 120 by a dry oxidation process in the trench 120. A nitride film 140 is formed over the resulting structure including the trench 120.
Referring to FIG. 1d, an insulating film 150 is filled in the trench 120 to form a device isolation film that defines an active region. The active region is etched to form a recess, and a radical oxidation process is performed on the resulting structure including the recess to form a gate oxide film.
In the conventional method for manufacturing a semiconductor device, after forming a trench, a void 155 is generated in a subsequent process because the oxide film formed by the dry oxidation process is not deposited with a uniform thickness in the trench. After the active region is etched to form a recess, the junction leakage is generated at sidewalls of the recess because of the gate oxide film formed uniformly in the recess by the radical oxidation process. As a result, it is difficult to make the refresh time of a memory device to be less than 50 nm.